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Thread: HELP: Finding the Percent Change

  1. #1
    Junior Member lillybeth's Avatar
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    Unhappy HELP: Finding the Percent Change

    Hello everybody!!! I need help with understanding how to find percent change.


    My book gives this formula: (amount of change/original amount) * 100%



    For a problem that looks like this: 80 to 100, I usually just do 20/80, which is equal to 25% (which my book's answer key tells me I am correct)

    Soooo.

    For a problem that reads 200 to 100, would you do 100/200, or -100/200??

    And one more thing:

    The formula our book tells us to use says to multiply the amount of change divided by the original amount by 100% (1.00).

    What is the point of multiplying by one??




    Thank you guys!!!
    All help will be greatly appreciated!!!
    Last edited by lillybeth; 10-11-2014 at 09:23 PM.
    "If you believe in something, believe in it all the way, implicitly and unquestionably." - Walt Disney

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    Quote Originally Posted by lillybeth View Post
    Hello everybody!!! I need help with understanding how to find percent change.
    My book gives this formula: (amount of change/original amount) * 100%

    For a problem that looks like this: 80 to 100, I usually just do 20/80, which is equal to 25% (which my book's answer key tells me I am correct)

    For a problem that reads 200 to 100, would you do 100/200, or -100/200??
    It is [tex]\dfrac{100-200}{200}=-\dfrac{100}{200}=-50\%[/tex].

    It is always [tex]\dfrac{\text{new price}-\text{old price}}{\text{old price}} [/tex].
    “A professor is someone who talks in someone else’s sleep”
    W.H. Auden

  3. #3
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    Percent means 'amount of parts out of 100 parts'. To use your example the change for 80 to 100 fractional change would be [tex]\frac{100-80}{80} = 0.25[/tex]. So for every one part you had, it would change by 0.25 parts. If you had a hundred parts, it would change by 100*0.25=25 parts which is 25 parts per hundred or 25%.

    Thus compute the fractional change, see pka's post, then multiple by 100 to get the percent change. That is what is meant in your equation of "multiply by 100%"

  4. #4
    Junior Member lillybeth's Avatar
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    Quote Originally Posted by pka View Post
    It is [tex]\dfrac{100-200}{200}=-\dfrac{100}{200}=-50\%[/tex].

    It is always [tex]\dfrac{\text{new price}-\text{old price}}{\text{old price}} [/tex].

    Thanks!!! But what about my second question??
    "If you believe in something, believe in it all the way, implicitly and unquestionably." - Walt Disney

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    Quote Originally Posted by lillybeth View Post
    Thanks!!! But what about my second question??
    We multiply by 100% - so that we can have the answer as "%" as opposed to a fraction or a decimal number (without %).
    “... mathematics is only the art of saying the same thing in different words” - B. Russell

  6. #6
    Junior Member lillybeth's Avatar
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    Cool

    Quote Originally Posted by Subhotosh Khan View Post
    We multiply by 100% - so that we can have the answer as "%" as opposed to a fraction or a decimal number (without %).
    Ok thx!!!!! I was just curious because there seemed no point really to multiply by one.

    Thank you soo much!!!

    "If you believe in something, believe in it all the way, implicitly and unquestionably." - Walt Disney

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